Future space missions may require vehicles traveling at relativistic velocities. Consequently, relativistic effects will have to be considered in the development of future space communication systems. This paper presents a formulation of these relativistic communication effects which are not normally encountered by the designer. The gain function, power density available at the receiver and apparent frequency of the source can be greatly affected by the motion of the space craft antenna. In addition, the apparent direction of the space craft can be quite different from its actual direction for both the transmitting and receiving conditions. Formulations are presented by which these effects are described for an arbitrary antenna in terms of the antenna characteristics in its rest frame and the actual space craft location, a form that is convenient for the designer. In addition, numerical examples are shown to illustrate the magnitude of these relativistic effects.